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Question

Difficulty: EasyMountain Driving and Steep Downgrades

When descending a steep mountain grade in a commercial motor vehicle, which braking method must the driver use to prevent brake fade and maintain a safe speed?

  1. A
    Maintain continuous, light brake pedal pressure to hold vehicle speed exactly at the posted safe speed throughout the entire descent.
  2. Apply the brakes firmly enough to feel a definite slowdown until speed reaches 5 mph5\text{ mph} below the safe speed, then release the brakes completely until speed returns to the safe speed.Answer
  3. C
    Shift the transmission into neutral and rely exclusively on the engine retarder to control downhill speed.
  4. D
    Pump the brake pedal rapidly with light, repetitive taps while attempting to downshift during the descent.

Answer

Apply the brakes firmly enough to feel a definite slowdown until speed reaches 5 mph5\text{ mph} below the safe speed, then release the brakes completely until speed returns to the safe speed.
The proper method for controlling speed down a steep mountain downgrade is snub braking: when the vehicle reaches its safe speed, the driver applies the service brakes firmly until speed drops 5 mph5\text{ mph} below safe speed (taking about 3 seconds), then releases the pedal completely. This cycle keeps the vehicle under control while giving the friction materials time to cool between applications.

Step-by-Step Solution

1
Select the correct low gear before cresting the hill and establish the maximum safe speed for the grade.
The engine provides maximum retarding power to help resist gravity acceleration.
Engine braking must serve as the primary speed control on steep descents to avoid overworking the service brakes.
2
When road speed reaches the designated safe speed, apply the service brakes firmly to reduce speed by 5 mph5\text{ mph} in approximately 3 seconds.
Vehicle speed drops 5 mph5\text{ mph} below the safe speed.
Firm application provides positive deceleration rather than continuous rubbing friction.
3
Release the brake pedal completely as soon as speed is reduced by 5 mph5\text{ mph}.
Brake linings and drums get a cooling period as the vehicle speed gradually creeps back up.
Releasing allows heat dissipation into the surrounding air, preventing brake fade.
4
Repeat this braking and release cycle for the duration of the descent.
The vehicle safely maintains speed within a 5 mph5\text{ mph} window without overheating the brake assemblies.
This cyclical process is the standard snub braking procedure required by the FMCSA CDL manual.

Key Concept

Proper snub braking technique on steep mountain downgrades
Estimated Time:45s
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Topics

Mountain Driving and Steep Downgrades

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